Numerous references teach using mixed matrix membranes which comprise a continuous polymer phase carrier with molecular sieves dispersed therein. Examples include U.S. Pat. No. 4,925,459 to Rojey et al. and U.S. Pat. No. 5,127,925 to Kulprathipanja et al. The membranes are particularly useful for separating gases from a mixture or feedstock containing at least two gas components, generally of differing effective diameters. Under the proper conditions, the molecular sieves may increase the relative effective permeability of a desirable gas component through the polymeric membrane (and/or decrease effective permeability of the other gas components), and thereby enhance the gas separation (selectivity) of the polymeric membrane material. If a mixed matrix membrane has a higher selectivity than a similar membrane without the molecular sieves, then the mixed matrix membrane is referred to as exhibiting a “mixed matrix” effect.
Membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desirable for membrane materials to have a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
U.S. Pat. No. 6,626,980 to Hasse et al., entitled “Mixed Matrix Membranes Incorporating Chabazite Type Molecular Sieves”, suggests that pore dimensions of molecular sieves are critical to the performance of membranes. The pore size determines whether molecules of a certain size can enter and exit the framework of a molecular sieve. Hasse et. al provide that, in practice, it has been observed that very slight decreases in ring dimensions defining such framework can effectively hinder or block movement of a particular gas component through a molecular sieve. Hasse et al. teach using a zeolite molecular sieve, SSZ-13, having a chabazite type structure for gas separation.
This chabazite type structure has pores based on 8 member rings with about 3.8×3.8 Angstrom dimensions. The synthesis of this particular SSZ-13 molecular sieve is disclosed in U.S. Pat. No. 4,544,538.
There is a need for additional choices of molecular sieves which have pores or pores therein which are sufficiently large so that permeability is satisfactory. If rates of permeation are insufficient, a membrane may not be economically viable for use in gas separation. Contrarily, if the selectivity of a membrane is not satisfactory, then the membrane again may not be economically viable as too much of a non-desirable gas component may also permeate through the membrane. The present invention provides mixed matrix membranes which utilize molecular sieves having structures, compositions and other characteristics which provide superior separation performance as compared to conventional membranes. Furthermore, methods of making and utilizing these membranes for gas separation are also taught.